Use of synthetic, iron raw materials for preparing iron oxide pigments

ABSTRACT

The invention relates to the use of synthetic (synthetically produced) iron raw materials to prepare iron oxide pigments in the Penniman process and other iron-dissolution processes.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 08/805,223 filed on Feb. 24, 1997, now U.S. Pat. No. 5,837,216.

The invention relates to the use of synthetic (synthetically produced)iron raw materials for preparing iron oxide pigments in the Penniman andother iron-dissolution processes.

Iron oxide coloured pigments, which are used as ecologically harmlesscolorants in ceramics, building materials, plastic materials, lacquersand paper, may basically be obtained in black, yellow, red and brownshades.

Iron oxide pigments are obtained, as described in Ullmann's Encyclopediaof Industrial Chemistry, VCH, Weinheim 1992, vol. A20, p. 298 et seq.,by solid phase reactions (red, brown and black pigments), precipitationand hydrolysis reactions of iron salts (yellow, red, orange and blackpigments) and by the oxidation of iron with aromatic nitro compounds inthe presence of hydrolysable, multi-valent salts (Laux process: DE-A 463773, DE-A 515 758).

Solid phase reactions are mainly used for the production of red ironoxides from black precursors (by calcination) or from FeSO₄ (Copperasprocess). Black precursors are produced by precipitation from solutionsof iron salts or by the Laux process. The production of iron oxide redpigments by roasting iron sulphate is an industrially complicatedprocess which is associated with several washing stages performed insequence, in which effluents which contain heavy metal are produced.

The production of yellow, orange, red and black iron oxide pigmentsusing precipitation (e.g. U.S. Pat. No. 2,388,659 for iron oxide yellowpigments) from solutions of iron salts and alkaline compounds in thepresence of air is associated with the disadvantage that stoichiometricamounts of neutral salts are produced, which appear in the effluent orhave to be worked up in a time-consuming and costly manner.

The Penniman process (U.S. Pat. No. 1,327,061 and U.S. Pat. No.1,368,748) reduces the amount of neutral salts which arises during theprecipitation process by using metallic iron as the raw material, thisbeing dissolved during the process by the acids being released.

The Laux process represents conversion of the well-known Bechampreaction to an industrial scale. By reducing aromatic nitro compoundswith metallic iron raw materials, the iron oxide arising in addition tothe aromatic amino compound, as an inevitable consequence of thereaction, is obtained in pigment quality by means of appropriatereaction management. The Laux process is a cost-effective andecologically harmless process for producing iron oxide pigments because,due to the use of metallic iron raw materials, no bases are required toprecipitate the iron oxide and therefore no neutral salts which have tobe disposed of are produced as by-products.

The Penniman and Laux processes therefore provide two cost-effective,ecologically harmless processes which use metallic iron as a rawmaterial for the direct production of iron oxide pigments.

The raw materials for these two processes are by-products from otherindustrial sectors; pickling liquors from the steel industry or ironsulphate from TiO₂ production are used as solutions of iron salts (e.g.FeCl₂, FeSO₄). The metallic iron raw materials are, in all cases,secondary raw materials from the metal processing industry (scrapmetal). Depending on the requirements for the process selected, thisscrap metal may be in the form of punched-out shapes, the tips of pinsor nails, sheet metal, bundles or turnings.

The use of this scrap metal to produce iron oxide pigments has severaldisadvantages because it is not a well-defined raw material. Thechemical composition of the scrap metal and also its reactivity in theprocesses may vary considerably.

It has therefore been suggested that the lack of and variations inquality associated with secondary raw materials be eliminated by the useof synthetically produced (synthetic) iron raw materials. Syntheticallyproduced iron raw materials are, for instance:

pig iron

cast iron

specular pig iron

steels

iron/steel powder.

In this case it is sensible, for economic reasons, to use only thoseiron raw materials which are produced in large amounts for otherindustrial sectors and are therefore available in sufficient amounts.

The synthetic iron raw materials selected should have the following setof properties:

1. suitable conveyance characteristics

2. ability to be metered out easily

3. complete dissolution during the process

4. low concentrations of doped and foreign metals

5. dissolution properties which are appropriate for the process and thereaction.

Since synthetic iron raw materials are available in the form of discretelumps (blocks, rods, billets, pigs, slabs, etc) or very finely divided(powders) or, for quality reasons, are alloyed, the use of commerciallyavailable, synthetic iron raw materials in iron-dissolution processesfor producing iron oxide pigments has not hitherto been disclosed. Lumpsof synthetic iron raw material are complicated to convey and to meter;in addition lumps of synthetic iron raw material dissolve only veryslowly and incompletely in the iron-dissolution processes used forpigment production. The foreign metal content of these iron rawmaterials, depending on the field of use in the further processingindustrial sector, can vary greatly. Depending on the particular fieldof use, chromium, molybdenum, vanadium and/or nickel contents of greaterthan 15 wt. % may be present.

Iron powder is easy to convey and meter out. Depending on the field ofapplication, however, high concentrations of foreign metals may bepresent in iron powders. Although iron powder dissolves completelyduring the course of iron-dissolution pigment-producing processes, dueto its reaction characteristics, no iron oxide particles with a pigmentcharacter are formed in iron-dissolution processes when using powders toproduce iron oxide pigments, as a result of the high reactivity. Theparticles obtained have an unsuitable size and size distribution andgenerally do not consist of a single phase.

Alloyed, synthetic iron raw materials, apart from inadequate reactivityin iron-dissolution processes, have the disadvantage that the alloyingelements are incorporated in the iron oxide particles. The incorporationof heavy metals, (e.g. Cr, Mo, V, Ni, Cu, etc) is associated with adecrease in the coloristic properties of the pigments obtained and isalso ecologically harmful.

Therefore no synthetic iron raw materials have hitherto been used iniron-dissolution processes, such as e.g. the Penniman process, forpreparing iron oxide pigments.

The problem was, therefore, to provide iron raw materials for preparingiron oxide pigments in iron-dissolution processes which do not have thedisadvantages of the prior art (inadequate metering and conveyancebehaviour, incomplete dissolution, heavy metal contents, low space-timeyields).

Surprisingly, it was found that specific, synthetic iron raw materials,in fact low-alloyed, spherical, approximately isometric, optionallyglobular or ellipsoid, iron particles, which, inter alia, are used asblasting agents, are suitable for producing iron oxide pigments iniron-dissolution processes and that they have advantages over bothsecondary raw materials and also other synthetic iron raw materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of MF versus run time for Example 1, Comparison Example1 and Comparison Example 2.

FIG. 2 is a plot of colour index Δb* versus MF for Example 1, ComparisonExample 1 and Comparison Example 2.

The invention provides the use of synthetic iron raw materials with anaverage particle size between 0.5 and 100 mm, with a metallic ironcontent greater than 80 wt. % for preparing iron oxide pigments iniron-dissolution processes whereby the iron raw materials are reactedwith oxidizing agents (e.g. air, oxygen or aromatic nitro compounds(nitrobenzene)) to form iron oxides or iron hydroxides or mixturesthereof in an acidic aqueous solution.

Spherical synthetic iron raw materials are particularly preferred.

The average particle size of the synthetic iron raw material ispreferably between 0.5 and 50 mm or between 20 and 100 mm (e.g. spongeiron).

The metallic iron content of the synthetic iron raw material ispreferably greater than 85 wt. %, in particular greater than 90 wt. %and specifically greater than 95 wt. %.

The foreign metal content of the synthetic iron raw material ispreferably less than 15 wt. %, in particular less than 10 wt. %,specifically less than 7 wt. % and most preferably 3 wt. %.

In general the synthetic iron raw materials, depending on the eventualapplication, are doped with the following foreign elements: Si, C, Cr,Ni, V, Mo, Cu, W, Ti, P, N, Co. These foreign metals may also combinewith the iron (e.g. C as Fe₃ C, cementite) and thus bond the iron.

The remainder (up to 100 wt. %) of the synthetic iron raw material,which is not present as metallic iron, iron-containing compound orforeign metal, may consist of iron oxides of various compositions (e.g.FeO, Fe₂ O₃, Fe₃ O₄).

The preferred, synthetic iron raw materials are characterised by thefollowing properties:

1. average particle size: 0.5-100 mm

2. particle size distribution: 80 wt. % of the particles are between onequarter and four times the mean value

3. metallic iron content: >80 wt. %

4. foreign metal content: <15 wt. %

Preferred synthetic iron raw materials have the following, beneficialproperties:

1. They are easy to convey.

2. They are easy to meter out.

3. They dissolve almost completely in iron-dissolution pigment-producingprocesses.

4. They have low foreign metal contents.

5. Their reaction characteristics are appropriate for pigment-producingprocesses.

Surprisingly, the reaction characteristics of the preferred iron rawmaterial is superior to the secondary raw materials used hitherto. Atthe same time, products with very favourable pigment properties areobtained, with improved space-time yields, in the Penniman and Lauxprocesses. Yellow pigments obtained in this way (see examples), forinstance, exhibit advantages in the mass tone lacquer test, with regardto the CIELAB colour index b* which is a measure of thequality-determining yellow fraction of the shade, as compared with thosewhich have conventionally been produced.

Masstone determination of the colour index for the pigments preparedusing synthetic raw materials and for the pigments from the comparisonexamples is performed in L 64 thixotropic paste (unhardened alkyd resinfrom Bayer AG) at a pigment volume concentration of 10%. Incorporationof the pigment into the paste is performed on a disc colour rubbing-inmachine with a 240 mm diameter with an applied load of 25 kg. Pigmentand paste are dispersed during 100 revolutions of the disc colourrubbing-in machine, during which time the apparatus is opened up and themixture collected together several times.

Colour evaluation of the pigments obtained according to the invention isperformed in Alkydal L 64 thixotropic paste (unhardened alkyd resin fromBayer AG) at a pigment volume concentration of 10%. The resultingpigmented paste is then analysed in a commercially available coloranalysing device of d/8° geometry (Ulbricht sphere). The reflectionfactors obtained are converted into the CIELAB color data system by thestandard method in accordance with ASTM E 308-85 and DIN 6174 (ISO7724/3, 1984; ASTM D 2244-85) using a C/2° standard illuminant, with theinclusion of surface reflection, C* being defined as the square root ofthe sum of the squares of a* and b* [C*=(a*² +b*²)^(1/2) ]. The relativecolour differences can be determined according to DIN 6174 or ISO 7724drafts 1-3, by comparing a sample with a designated reference substance.

The relative colour differences of the pigment samples are calculatedaccording to DIN 6174 (ISO 7724) against a reference material preparedusing conventional secondary raw materials.

The metallic iron content of the iron raw material used is determinedusing the hydrogen method:

1. Equipment

Gas generator: 250 ml round-bottomed flask, 50 ml measuring droppingfunnel with gas take-off pipe and gas cap for nitrogen feed-line, 1000ml gas collecting vessel (measuring dropping funnel) with graduationsand tap, vacuum connection, pneumatic tank (31) with thermometer, 500 mlmeasuring cylinder, heating mantle for 250 ml round-bottomed flask.

2. Method

A pneumatic tank is filled with water. About 2 g of the sample ofsynthetic iron raw material to be determined is weighed into a 250 mlround-bottomed flask. After adding 250 ml of distilled water, theround-bottomed flask is placed in the heating mantle and a measuringdropping funnel with gas take-off pipe is inserted. The contents of theround-bottomed flask are heated to boiling point and maintained at thattemperature. As soon as no more gas bubbles escape at the tip of the gastake-off pipe, 40 ml of dilute hydrochloric acid (about 10% strength) isplaced in the dropping funnel, the dropping funnel is sealed with a gascap and the gas take-off pipe is introduced, from underneath, into thegas collecting vessel. The end of the nitrogen pipe is pushed into a 500ml measuring cylinder which is about 2/3 full of water, so that acounter-pressure is thereby produced. The gas collecting vessel iscompletely filled with water by applying a vacuum. The hydrochloric acidis added dropwise to the round-bottomed flask. The solution in theround-bottomed flask is heated so that it boils gently, until theevolution of hydrogen has terminated.

3. Experimental Values Which Are Measured

Volume of the water displaced in the gas collecting vessel (V)

Difference between level of water in the gas collecting vessel and inthe tank (h)

Temperature of water (T_(w))

Room temperature (T)

Air pressure (P_(air))

4. Calculation

Pressure (P)=P_(air) -P_(hydrostat) -P_(H2O)

P_(air) =air pressure; P_(hydrostat) =h×0.0736 (hydrostatic pressure)

P_(H2O) =pressure of water vapour (see tabulated data)

normalised volume (V^(o)): P×V×273/R×T×760

Content (Fe_(met).): 2.000 g weighed out corresponds to 803.3 ml ofhydrogen

(for 100% metallic iron) metallic iron content in the sample ofsynthetic iron raw material:

    Fe.sub.met. =V.sup.o ×100/803.3×(weight of sample/2)

The invention will be explained in more detail by means of the followingexamples. The examples are intended as an illustration of the use ofsynthetic raw materials in pigment preparing processes which are knownper se and do not represent any restriction. The following describes theproduction of iron oxide yellow pigments which are produced underconditions known to a person skilled in the art (pH<4.5) iniron-dissolution processes (Penniman and/or Laux process). The phaseformation of iron oxide pigments in iron-dissolution processes ispH-controlled, in the acid pH region (<4.5), α-FeOOH is formed, in theweakly acid region (about 4.5-5.5), γ-FeOOH and α-Fe₂ O₃ are producedand in the pH region >5, the formation of Fe₃ O₄ is preferred, whereinit is left to a person skilled in the art to decide the preciseconditions under which γ-FeOOH, α-Fe₂ O₃ or Fe₃ O₄ are produced whenusing the synthetic iron raw materials.

Penniman Process

1. Description of the Equipment

9.2 l stainless steel pot with a gas distribution ring located justabove the base, a vane stirrer, a control thermoelement, a pH electrode,a glass lid and a reflux condenser.

2. Description of Nucleation

Enough 25% strength caustic soda solution is added with stirring to anaqueous solution of iron(II) sulphate with 150 g/l of FeSO₄ for 40% ofthe iron to be precipitated as iron(II) hydroxide. By aerating with airat 35° C., Fe(II) is oxidised to Fe(III); the pH then falling from 7 to3.

3. Description of the Pigment Build-up Reaction

Nuclei used: α-FeOOH with a specific surface area of 59 m² /g. 9 l ofnucleus-suspension with a concentration of 7.0 g/l of α-FeOOH and aFeSO₄ content of 40 g/l, as well as 650 g of metallic iron raw material,are initially introduced. The suspension is heated to 85° C. withstirring at 200 rpm and then aerated with 200 l/h of air with furtherstirring. After specific oxidation times, samples of on average 250 mlare withdrawn. Reaction is terminated after 82 hours.

The α-FeOOH content of the samples is determined. From the α-FeOOHcontent after specific oxidation times and the α-FeOOH nucleus contentused, the multiplication factor MF is calculated in accordance with thefollowing equation:

    MF=α-FeOOH content (of sample)/α-FeOOH content (of nuclei)

The following Table characterises the metallic, spherical iron rawmaterials used in the examples:

                  TABLE 1                                                         ______________________________________                                        Characteristics of the metallic, spherical iron raw materials                                                      Foreign                                                                       metal                                    Shape         Particle sizes                                                                             Metallic iron                                                                           content                                  ______________________________________                                        Com-   Irregularly                                                                              Area: 10-50 cm.sup.2                                                                       98%     <10%                                   parison                                                                              shaped bits of                                                                            Thickness: 1-3 mm                                          example 1                                                                            sheet metal                                                            Com-    Irregularly                                                                                Area: 10-50 cm.sup.2                                                                         94%                                                                                    <10%                             parison                                                                              shaped bits of                                                                            Thickness: 0.3 mm                                          example 2                                                                             sheet metal                                                           Example 1                                                                             globular and                                                                               0.25-4 mm               <10%                                    ellipsoidal                                                            ______________________________________                                    

The run times for sample withdrawal, the multiplication factors (MF) andthe CIELAB colour index Δb* measured are listed in the following Tablefor the individual examples. The reference material used was the pigmentbuilt up in the same way as the pigments prepared according to theinvention (MF=25.5), but which had been obtained using conventional rawmaterials.

The samples are filtered by conventional methods, washed salt-free anddried for 15 hours at 90° C. in a drying cabinet. Then the CIELAB colourindices are determined in accordance with the methods given above.

                  TABLE 2                                                         ______________________________________                                        Run times, MF and CIELAB color index Δb* for comparison                 examples 1 and 2 and example 1.                                               ______________________________________                                        Comparison                                                                             example 1        Comparison                                                                            example 2                                   run time [h]                                                                             MF             Δb*                                                                          run time [h]                                                                     MP              Δb*                   ______________________________________                                        37                        36                    1.4                           43                        40                    1.3                           46                        44                    1.1                           61                      0.8                                                                                  61               0.7                           79                      0.3                                                                                  68               0.6                           89                      0.0+                                                                                 80               0.5                           ______________________________________                                                        Example 1                                                     Run time [h]                                 Δb*                        ______________________________________                                        39              14.4     1.7                                                  45                                         1.5                                52                       1.1          20.0                                    59                                         0.8                                67                                         0.6                                71                                         0.6              25.4              73                                         0.6                                ______________________________________                                         += reference material                                                    

FIGS. 1 (MF against run time) and 2 (Δb* against MF) show that thesynthetic, metallic iron raw material has advantages over the prior art(conventional scrap sheet metal) with regard to space-time yields andalso, at comparable space time yields, over scrap sheet metal withregard to the quality-relevant CIELAB colour index b*.

Laux Process

The use of synthetic iron raw materials for producing iron oxide yellowpigments by the Laux process is described by way of example in thefollowing examples, wherein a person skilled in the art may vary theselection of industrial aggregates or additional feedstocks, dependingon requirements. Iron oxide pigments which are obtained by the reactionof metallic iron raw materials with nitrobenzene are prepared inaccordance with the Laux process which is described in U.S. Pat Nos. DE463,773, DE 464,561, DE 515,758 and 4,234,348 (Ullmann's Encyclopedia ofIndustrial Chemistry Vol A20, p. 301-303 and 362, 1962), the completedisclosures of which are expressly incorporated herein by reference. Byvarying the additional feedstocks, a person skilled in the art canspecifically prepare any of the iron oxide phases obtainable in the Lauxprocess (α-FeOOH, γ-Fe₂ O₃, α-Fe₂ O₃ or Fe₃ O₄), as is generally known.Using the process described in the Patents, the following substances areconverted in pressure-resistant tanks fitted with stirrers.

                  TABLE 4                                                         ______________________________________                                        Substances and process parameters for comparison example 3 and                example 2 (size of tank: 2 liters)                                            Parameter         Comp. example 3                                                                           Example 2                                       ______________________________________                                        1st phase                                                                             FeCl.sub.2 solution [1]                                                                     0.12        0.124                                                                         0.05       H.sub.2 O [1]                                                                 0.046                                                  0.055                 0.055sub.3 (16O g/l) [1]                                  0.08                 0.08llic iron raw                                                             material [kg]                                             0.095              0.095obenzene [1]                 2nd phase                                                                                    metallic iron raw                                                                      0.352               0.568                                                                          material [kg]                                             0.240              0.408benzene [1]                                                    0.6        H.sub.2 O [1]                                                                  0.6                                                        7                    6time up to                                                                100% conversion                          [h]                                                                   ______________________________________                                    

The metallic iron raw materials used in the examples are characterizedin the following table.

                  TABLE 5                                                         ______________________________________                                        Characteristics of the metallic iron raw materiais used                                                            Foreign                                                               Metallic                                                                              metal                                    Shape         Particle sizes iron    content                                  ______________________________________                                        Compa- Pin and nail-                                                                            Length: 10-50 mm                                                                             98%   <10%                                   rison                                                                         example 3                                                                                shaped lumps                                                                         Thickness: 0.5-1 mm                                                                                 of metal                              Example 2                                                                               globular and                                                                          0.25-4 mm                 <10%                                                                      ellipsoidal                           ______________________________________                                    

Working Up the Samples

In a 300 ml beaker, 50 ml of freshly distilled aniline is added to about200-250 g of pigment paste, stirred with a paste spoon and then theaniline is decanted off. The process is repeated until the aniline isclear and colourless after being stirred into the paste. Then the washedpaste is transferred to a 2 liter wide-necked flask and slurried withdrinking water, with vigorous shaking. To separate excess iron and thecoarse fraction, the suspension is passed through a 40 μm sieve. Thesuspension obtained is filtered under vacuum through a Buchner filterwhich is lined with filter paper. As soon as the suspension has beensuction filtered, about 3-4 portions of 250 ml of drinking water arepoured in sequence over the paste on the filter in order to wash outsalts.

The sludge-like pigment paste is dried for 1 hour at 170° C. in a dryingcabinet. The dried pigment is brushed through a 9 mesh sieve and thenincorporated into the binder system (L 64 thix. see above).

The pigment which was obtained by using conventional secondary rawmaterials was used as the reference material.

                  TABLE 6                                                         ______________________________________                                        Run times up to 100% nitrobenzene conversion and CIELAB color                 index Δb* for comparison example 3 and example 2                        Run times to                                                                  100%          Weight of           CIELAB                                      nitrobenzene  pigment   Space-time                                                                              color index                                 conversion [h]                                                                              obtained [g]                                                                            yield [g/l.h]                                                                           Δb*                                   ______________________________________                                        Comp.  7          563       40.2    0.0+                                      example 3                                                                                                         (+ = ref. mat.)                           Example 2                                                                               6                                9.0                                ______________________________________                                    

The results which are given in Table 6 show that the use of syntheticmetallic iron raw materials in the Laux process also has advantages overthe use of conventionally used raw materials, these being linked to thespace-time yield and also to the quality-determining CIELAB colour indexb*.

What is claimed is:
 1. A method of using synthetic iron raw materialsconsisting essentially of spherical iron particles with an averageparticle size between 0.5 and 100 mm and a metallic iron content greaterthan 80 wt. % to prepare iron oxide pigments in iron-dissolutionprocesses, wherein said method comprises reacting said iron rawmaterials with oxidizing agents to form iron oxides or iron hydroxidesor mixtures thereof in an acidic aqueous solution.
 2. A method accordingto claim 1, wherein said iron raw material has an average particle sizeof between 0.5 mm and 50 mm.
 3. A method according to claim 1, whereinsaid iron raw material has a metallic iron content of greater than 85wt. %.
 4. A method according to claim 1, wherein said iron raw materialhas a metallic iron content of greater than 90 wt. %.
 5. A methodaccording to claim 1, wherein said iron raw material has a metallic ironcontent of greater than 95 wt. %.
 6. A method according to claim 1,wherein said iron raw material has a foreign metal content of less than15 wt. %.
 7. A method according to claim 1, wherein said iron rawmaterial has a foreign metal content of less than 10 wt. %.
 8. A methodaccording to claim 1, wherein said iron raw material has a foreign metalcontent of less than 7 wt. %.
 9. A method according to claim 1, whereinsaid iron raw material has a foreign metal content of less than 3 wt. %.